1. Field of the Invention
The present invention relates generally to materials for use in the manufacture of creped fibrous webs. More particularly, it relates to a formulation which reduces or eliminates blocking.
2. Description of the Related Art
Inventors in the prior art have developed methods for providing strength and abrasion resistance to the surface of a fibrous web while at the same time increasing the softness, absorbency and bulk of the web. U.S. Pat. No. 3,879,257, granted Apr. 22, 1975 to Gentile et al, discloses a unitary or single-ply fibrous web having a laminate-like structure which consists of a soft, absorbent central core region of relatively low fiber concentration sandwiched between two strong, abrasion resistant surface regions. According to Gentile et al, the surface regions are formed by application of a bonding material, preferably elastomeric. The bonding material in at least one surface region is disposed in a fine, space-apart pattern, and the bonded portions in that surface region are finely creped to soften them. According to Gentile et al, the bonding material adheres the web in a pattern to a creping surface from which it is removed by a doctor blade to greatly disrupt the fibers within the web where bonding material is not disposed. This "bonding material", which adheres the web to the creping surface in a controlled manner, is variously referred to in the art as " binder", "adhesive" or more particularly "creping adhesive".
Where the present invention has its greatest utility in processes derived from the teachings of Gentile et al, the term "creping adhesive" will be employed herein to describe a bonding material which performs the functions of adhering a fibrous web to a creping surface and imparting strength to the creped sheet by bonding the fibers together. As thus broadly defined it encompasses materials applied to the surface of the web or to the creping surface.
The concepts of Gentile et al were extended to the formation of laminates of two or more dry webs in Salmeen et al, U.S. Pat. No. 4,610,743, granted Sept. 9, 1986. According to Salmeen et al, as in Gentile et al, both surface regions of the resulting product have creping adhesive disposed therein. Another variation on patterned creping of fibrous webs is disclosed by Klowak et al, U.S. Pat. No. 4,125,659 granted Nov. 14, 1978. According to Klowak et al, a creping adhesive is uniformly applied to just one side of the web and after the web has been adhered to the creping cylinder, a raised pattern is pressed into the web toward the creping cylinder. While all of these patented processes offer substantial improvements in strength and absorbency, the products can suffer from a serious disadvantage, not mentioned in any of the patents, which is "blocking".
The term "blocking" refers to the tendency of adhesively bonded webs or sheets to adhere to one another when in contact under pressure such as when wound up on a reel or when packaged in rolls or stacks. This negative characteristic interferes with the conversion of parent rolls and the dispensing and unfolding of the finished product. The tendency of creped web to "block" is proportional to the amount of creping adhesive on the surface of the web and thus is more severe in the case of pattern-creped, adhesively bonded materials such as described by Gentile et al and others mentioned above.
The phenomenon of blocking of creped fibrous sheets is specifically addressed by Grossman in U.S. Pat. No. 4,063,995, granted Dec. 20, 1977. The term is defined therein at Col. 1, lines 58-66. Grossman teaches the use of a four component binder and creping adhesive composition comprising, inter alia, a blocking suppressant. Grossman describes a variety of materials as being suitable blocking suppressants among them being any of the known polymers suitable for use as a binder for fibers "provided it has a glass transition temperature of from about +30.degree. C. to about +50.degree. C. so that it will function as a blocking suppressant." Col. 6, lines 17-22.